Plastic lenses, as compared with glass lenses, are characterized by being light-weight and hard to break. As a result they have recently been widely used for optical devices and instruments, such as eye-glass lenses and camera lenses. Particularly, plastic lenses for eye-glasses have been rapidly increasing their market share owing to increased refractive index and decreased specific gravity of raw material resins.
As for material resins, diethylene glycol bisaryl carbonate resin has been used for many years. In recent years, urethane type resin has been developed which is obtained by reaction between a polyisocyanate compound and a polyol or polythiol compound.
As for methods of manufacturing polyurethane lenses, there are known those which utilize the reaction between a polyisocyanate compound and a polyol compound proposed in JP-A-57136601, the reaction between a polyisocyanate compound and a polyol compound which contains halogen atoms proposed in JP-A-58164615, the reaction between a polyisocyanate compound and a polyol compound having a diphenylsulfide chain proposed in JP-A-60194401, the reaction between a polyisocyanate compound and a polyol compound which contains sulfur atoms proposed in JP-A-60217229, and the reaction between a polyisocyanate compound and polythiol in which a hydroxyl group in polyol is replaced by a mercapto group proposed in JP-A-60199016, JP-A-62267316, JP-A-63046213, and JP-A-05320301. Production of these polyurethane lenses is usually effected by the casting polymerization method.
Conventionally, in casting polymerization, as shown in FIG. 5, molds 4 and 4-1 and a gasket 5 are used to constitute a lens producing form 8 called a shell. After this form 8 has been clamped by a spring 6, a resin precursor 7, namely a monomer or the like, which is a polymerizable material, is injected into the internal space to initiate polymerization while the shell is kept stationary in the heating furnace.
In the case where plastic lenses are produced by this casting polymerization, there is proposed a method of manufacturing lenses in JP-A-05212732 which comprises the steps of separately mixing polyisocyanate and polyol with various additives to form uniform mixed solutions, separately degassing the solutions, continuously mixing the two solutions together while injecting the mixture into a lens producing form and, allowing it to undergo casting polymerization to produce a foamless, optically superior lens.
Further, when plastic lenses are to be produced by casting polymerization, the polymerization temperature and the duration of polymerization are important. It is stated, for example, in JP-A-05-212732 that the initial temperature is preferably 5-50.degree. C. and it is raised to 100-140.degree. C. in 5-50 hours, and that if the initial temperature is less than 50.degree. C., the polymerization takes a longer time, while if it is higher than 50.degree. C., the resulting lens is liable to be optically inhomogeneous.
Even if the above points are taken into account, however, with the conventional method of manufacturing plastic lenses by casting polymerization requiring the shell to be kept stationary in the heating furnace, the optical strain called stria cannot be completely eliminated, thus, impeding the increase of yield and making cost reduction hard to achieve.
Though the reason why the optical strain occurs in the polymerization process is not clear, it seems that when polymerization proceeds under heat from the outside, a reaction gradually takes place in the monomer in the interior of the molding form and as the monomer polymerizes to become an olygomer, the specific gravity changes. The variation in specific gravity causes the traveling of the monomer and olygomer, and the trail of the travel forms a cause of striae.
Based on the above consideration, we have studied how to prevent the traveling of the monomer and olygomer and have finally developed a method of manufacturing plastic lenses which have no or very little optical strain.
The problems the invention is intended to solve is to prevent the occurrence of optical strain in plastic lenses produced by the casting polymerization method.